The present invention relates to an optical module which is applied to a wavelength-division multiplexing optical communication system.
In fiber optic communications, in recent years there has been an increasing demand for a greater transmission capacity. For this reason, a wavelength-division multiplexing optical communication system in which light having different wavelengths is transmitted by one optical fiber to increase the transmission capacity has been put to practical use. The wavelengths used in such a wavelength-division multiplexing optical communication system are limited to bands in the vicinities of the wavelength of 1.3 or 1.55 μm. For this reason, if the intervals between adjacent wavelengths can be made narrower, the number of transmittable wavelengths can be increased. The wavelengths that may be used have been standardized in accordance with the recommendation by International Telecommunication Union (ITU). The spacing of the wavelengths used is tending to become narrower from 200 GHz to 100 GHz and further to 50 GHz, and these frequency spacings, if expressed by the wavelength interval, correspond to 1.6 nm, 0.8 nm, and 0.4 nm. To make the wavelength interval narrower, it is necessary to stabilize each wavelength with high accuracy, and the demand for optical modules for stabilizing the oscillation wavelength of a semiconductor laser has been increasing as a light source therefor.
In an optical module in which the oscillation wavelength of a semiconductor laser is stabilized, a number of methods have been proposed for stabilizing the wavelength by monitoring it. As a method in which a bandpass filter for transmitting a specific wavelength of the laser beam is used, entitled “Wavelength-Variable Laser Light Source Device JP-A-2000-012968 for example, is known. Further, as a method of detecting and stabilizing the wavelength by converting laser light into diffused light by a lens and causing the diffused light to be incident upon an etalon substantially orthogonally thereto, for example, Japanese paper, Sakai et. al. “Tunable LD Module with Wavelength Detector Using Etalon (Proceedings of the 2000 Japan IEICE General Conference (C-3-143))” is known.